1. Field of the Invention
This invention relates to a process for preparing halogenated diaryl hydrogen phosphate esters. More particularly it relates to the hydrolysis of tris(2,6-dihalophenyl) phosphates, and to reaction of certain 2,6-dihalophenols with phosphorous oxyhalide and hydrolysis of the reaction products, to form diaryl hydrogen phosphates having in admixture only a minor proportion of monoaryl and triaryl phosphates.
Halogenated aryl phosphate esters are useful as plasticizers, pesticides, flame retardants, and textile impregnating agents. As flame retardants they may be incorporated into plastic such as polyethylene and polystyrene, or they may be applied to fabrics from solution in water or an organic solvent. Halogenated diaryl hydrogen phosphates are currently of interest both as flame retardants and as intermediates in the preparation of diaryl phosphate derivatives.
2. Description of the Prior Art
The preparation of aryl phosphates by reaction of phenols (ROH) with phosphorous oxyhalides (POX.sub.3) and subsequent hydrolysis has long been known. Although the process has been considered satisfactory for the preparation of mixtures of monoaryl, diaryl, and triaryl phosphate esters, it has not been recommended for the production of acceptably pure diaryl hydrogen phosphate because of the difficulty of separating the mixed esters produced by the process. If two moles of a phenol are reacted with one mole of phosphorous oxyhalide and the reaction products are hydrolyzed, the product has been reported to be not primarily the desired diaryl hydrogen phosphate, but rather a mixture of monoaryl, diaryl, and triaryl phosphates together with unreacted phenol. The yield of the desired reaction EQU 2ROH+POX.sub.3 .fwdarw.(RO).sub.2 P(O)X+2HX
is diminished by competition from the reactions EQU ROH+POX.sub.3 .fwdarw.ROP(O)X.sub.2 +HX
and EQU 3ROH+POX.sub.3 .fwdarw.(RO).sub.3 PO+3HX.
the mixture of products from these reactions is difficult to separate. Hydrolysis of the mixture yields a mixture of esters which is also difficult to separate. The process is said to produce good yields of triaryl phosphate if an excess of the phenol, i.e., more than three moles per mole of phosphorus oxyhalide, is used.
A number of methods have been proposed for the preparation of pure diaryl hydrogen phosphates. U.S. Pat. No. 3,019,249 describes the use of an exchange reaction between triaryl phosphate and a selected proportion of phosphoric acid or alkali metal phosphate.
U.S. Pat. No. 3,155,706 relates to the preparation of diarylphosphorohalidates which may be hydrolyzed to form diaryl hydrogen phosphates, by adding, separately and simultaneously, one molecular proportion of phosphorus oxyhalide and one molecular proportion each of an alkali metal hydroxide and an alkali metal phenoxide in aqueous solution to one molecular proportion of a phenol while maintaining the temperature of the reaction mixture between -10.degree. C. and 20.degree. C. and the pH between 4.0 and 6.9. The need for careful control of rates of addition, temperature, and pH is evident. This patent also states that selective hydrolysis of triaryl phosphate esters to diaryl esters is difficult to control, particularly in acidic conditions, where progressive degradation products down to phosphoric acid generally are formed.
U.S. Pat. No. 3,657,398 shows the preparation of halogenated diaryl phosphates including bis(2,4,6-tribromophenyl) hydrogen phosphate by reaction of a halogenated phenol with an alkyldichlorophosphate in the presence of a catalyst such as tertiary amine.
U.S. Pat. No. 3,689,602 shows the preparation of halogenated aryl phosphates by reaction of halophenols including 2,4,6-tribromophenol with acid halides of phosphorus, including phosphorus oxychloride, in the presence of a catalytic amount of a tertiary aromatic amine or a carboxylic acid amide.
The cited patents point out shortcomings of the phenol-phosphorus oxyhalide-hydrolysis route to diaryl hydrogen phosphates and emphasize that a mixture of reaction products is obtained.